In sublimation type or molten type thermal transfer recording, a film of polyethylene terephthalate (PET) is generally used as the base material for the ink sheet, but since the heat resistance and running lubricity of the PET film are insufficient when used in the recording head such as thermal head, a heat resistant lubricating layer having heat resistance and lubricity is usually formed and used at the base material side of the ink sheet in contact with the recording head.
As such heat resistant lubricating layer, it has been widely proposed to use substances having polysiloxane as constituent component, for example, a vinyl polymer by block or graft coupling of organopolysiloxane units as disclosed in Japanese Laid-open Patent No. 61-143195, silicone denatured urethane resin as disclosed in Japanese Laid-open Patent No. 62-82086, stick-preventive layer made of silicone graft polymer as disclosed in Japanese Laid-open Patent No. 1-214475, and a heat resistant protective layer of amino denatured polysiloxane and polyisocyanate or reaction product of both as disclosed in Japanese Laid-open Patent No. 5-131770.
The component for providing the heat resistant lubricating layer with lubricity is, for example, a silicone graft polymer is a polymer having silicone grafted in a polymer, and therefore it is low in the content of free silicone such as liquid silicone and the silicone content is not so much although some have film forming capability, and a sufficient lubricating performance is not obtained.
Hitherto proposed liquid silicone oils such as amino denatured polysiloxane are superior to silicone graft polymer in lubricity, but not sufficient, and to obtain a required lubricity, it is necessary to increase the adding amount relatively, and moreover since the molecular weight is not so large, when the ink sheet is stored in the condition of high temperature and high humidity in rolled state, silicone oil is likely to diffuse and move from the heat resistant lubricating layer side into the color material layer side which rolls and contacts with the heat resistant lubricating layer, and, as a result, the lubricity of the heat resistant lubricating layer is lowered after storage, and creases are likely to be formed when recording.
In sublimation type thermal transfer recording, usually, the ink sheet and image-receiving sheet are joined together between the recording head and platen, the back side of the ink sheet is heated by the recording head heated according to the recording signal, and the dye in the color material layer of the ink sheet is transferred in the dye image-receiving layer of the image-receiving sheet by sublimation or diffuse transfer process, thereby forming a recording image in the image-receiving sheet. In this recording process, the color material layer of the ink sheet and the dye image-receiving layer of the image-receiving sheet receive a larger thermal energy from the recording head in the face-to-face contact state, and the heat resistance of the high polymer used in the color material layer and dye image-receiving layer cannot be set too high for the ease of migration of the dye in the color material layer into the dye image-receiving layer, and therefore thermal fusion is likely to occur on the contact surfaces of both layers. To prevent thermal fusion, generally, various parting agents are applied on both layers.
Incidentally, when recording by a printer, feed of plural image-receiving sheets is likely to occur in the automatic sheet feeder in which multiple image-receiving sheets are set, and to solve this problem, the back side of the image-receiving sheets is coated with various lubricants to improve running performance of the image-receiving sheets.
Proposals about the ink sheet include, for example, the parting polymer contained in the color material layer of the ink sheet as disclosed in Japanese Laid-open Patent No. 7-164768, proposals about the image-receiving sheet include, for example, the use of polyoxy alkylene silicone copolymer in the dye image-receiving layer of the image-receiving sheet as disclosed in Japanese Laid-open Patent No. 61-244589, the use of reaction curing type silicone oil as disclosed in Japanese Laid-open Patent No. 61-237694, and the use of silicone particles as disclosed in Japanese Laid-open Patent No. 1-176588.
Among the hitherto proposed polyoxy alkylene silicone copolymers, in particular, the non-reaction type parting agent is excellent in prevention of thermal fusion, but involves a new problem. That is, the ink sheet is fabricated, and it is turned in the process until finally wound in the cassette, and is also turned when slitting, and thus several turning operations are done. In the turning operations, when a non-reaction parting agent is used in the color material layer of the ink sheet, since the conventional parting agent is not so large in molecular weight, the parting agent in the color material layer of the ink sheet is likely to be transferred on the back side of the ink sheet, and hence the dye is also likely to be transferred in the parting agent transferred on the back side. Or, since the dye is dissolved in the parting agent, as the parting agent is transferred, the dye is also transferred. As a result, after finally winding in the cassette, when the ink sheet is rolled and stored in the condition of high temperature and high humidity, the dye transferred on the back side of the ink sheet is further transferred again on other color material layer of the surface, which causes a problem of contamination of other color material layer with dye. On the other hand, in the case of reaction curing type silicone oil, it is excellent in thermal fusion preventive characteristic, and the content of free silicone oil of low molecular weight is small, and hence the problem of contamination with dye is rare, but the paint containing expensive dye used in forming of color material layer is of reaction curing type, and is hard to be recycled. Silicone particles are effective against contamination with dye, but are not sufficient in thermal fusion preventive characteristics.
Similarly, when added to the dye image-receiving layer of the image-receiving sheet, the conventional polyoxy alkylene silicone copolymers, mainly non-reaction type parting agents, are not so large in molecular weight, and the parting agents are hence likely to be released from the dye image-receiving layer forming resin. When the image-receiving sheet having recording image is stored at high temperature and high humidity, part of the recording image dye is released together with the parting agent, and if touched by hand after storage, part of the recorded dye is transferred again on the hand, and thus the fixing of the dye may be lowered. The reaction curing type silicone oil is rare in the problem in storage at high temperature and high humidity, but same as the color material layer, it is hard to recycle the dye image-receiving layer forming paint. When silicone particles are used in the dye image-receiving layer, too, the thermal fusion preventive performance is not excellent.
When a plurality of image-receiving sheets having recording images are joined together and stored in the condition of high temperature and high humidity, the dye of the recording image is similarly transferred again on the resin layer on the back side of the image-receiving sheet having the dye image-receiving layer formed on the surface, and the back side of the image-receiving sheet may be contaminated.